Based on previously reported individuality of serum chemical and hematological constituents uniquely characterizing a person's state of health, a working hypothesis was conceived that the individual profile of quantitative measurements as defined by a time-series of values over a period of optimum health, constitutes a reference state. This persists during optimum health. A departure (deviation beyond the reference limits) of one or more characteristics or a composite index, although within the conventional "normal range" may not only predict development of abnormality but signal the initiation of preventive measures to reverse the detected trend. The establishment of individual reference profiles, periodic monitoring and appropriate intervention may constitute a most sensitive and effective approach to health maintenance and preventive medicine. This study is designed to test this hypothesis and answer related questions including: 1. What are relevant chemical, physiological and psychological measurements? 2. What are adequate numbers and time patterns of measurements for reference profiles? 3. What are definable criteria of optimum health? 4. What are optimum intervals for monitoring an individual's changing health? 5. What are criteria for recognizing "departures? 6. How can departures be interpreted in relation to extrinsic (environmental) factors and subsequent development of abnormality? 7. What measures of tolerance to challenge and stress are relevant to the individual reference profile and of predictive sensitivity? These require long-term (15-20 years) studies of initially healthy people; the development of biomathematical models of computer processed data, innovative statistical analyses and automated, precise and cost- effective testing procedures. Bibliographic references: Interlaboratory comparison of enzymatic methods for serum glucose determination, N. Gotchman, W. Ryan, R. Sterling, and G. Widdowson, Clinical Chemistry, 21:356, (1975). Performance specifications for instruments used in enzyme activity measurements", Graham M. Widdowson, Second International Symposium on Clinical Enzymology, October, 1975, Mount Sinai Hospital Medical Center, Chicago.